Meeting Abstract
Phenotypic flexibility, or the ability to make reversible phenotypic adjustments to variation in environmental conditions, is essential for organisms to cope with environmental change. An emerging framework for understanding variation in phenotypic flexibility is the Climate Variability Hypothesis (CVH), which predicts that flexibility should increase with climatic variability. For example, thermal tolerances of both ectotherms and endotherms increase with latitude (a proxy for climatic variability), which has been interpreted as support for the CVH. However, whether a) thermal tolerance is a flexible trait that can be adjusted to changing environmental conditions, and b) flexibility in thermal tolerance increases with climatic variability as predicted by the CVH, remains unclear. We used flow-through respirometry to measure seasonal variation in breadth of the thermoneutral zone (TNZ; a proxy for thermal tolerance) in a suite of temperate-zone and tropical bird species to determine whether avian thermal tolerance is a flexible trait that varies seasonally and whether temperate-zone birds exhibit greater seasonal flexibility in thermal tolerance than their tropical counterparts. We found that TNZs of tropical species were relatively invariable across seasons (wet vs. dry), whereas TNZs of temperate-zone species were flexible and increased with increasing climatic variability (i.e. winter-acclimated individuals had significantly broader TNZs than summer-acclimated conspecifics), supporting the CVH. Avian thermal tolerance is thus a flexible trait and the magnitude of flexibility varies across latitude, suggesting that tropical birds may have reduced ability to cope with environmental change compared to temperate-zone counterparts.
